Shortgrass steppe and northern mixedgrass prairie plant species traits

Despite progress in trait-based ecology, there is limited understanding of the plant traits that structure semiarid grasslands. In particular, it remains unclear how traits that enable plants to cope with water limitation are related to traits that influence other key functions such as herbivore defense and growth. The hypothesis that drought and herbivory exert convergent selection pressures is supported for morphological traits, but largely untested for struct­ural, physiological, and phenological traits. Drought and economic traits can also covary, but where and to what degree remains uncertain.

Here we address these uncertainties in semiarid shortgrass steppe and mixedgrass prairie, the largest remaining grasslands in North America. Using a broad selection of traits for 37 of the most common plant species in each ecosystem, we ask whether traits that confer drought tolerance, avoidance and escape covary with herbivore resistance traits and economic traits.

Results reveal that both drought tolerance and escape are coordinated with other functions, but in opposite fashion. Drought tolerant species (low leaf osmotic potential and high leaf dry matter content, LDMC) were also herbivore resistant (high leaf toughness and cellulose) and at the ‘slow’ end of the economic spectrum (low leaf nitrogen, leaf phosphorus, and high stem density). Conversely, drought escape via early senescence was associated with lower drought tolerance, lower herbivore resistance, and ‘fast’ economic traits. Drought avoidance, as indicated by thick leaves, may also be associated with lower drought tolerance (LDMC). Senescence date and LDMC appear to be key traits in these semiarid grasslands, differentiating species along multiple axes of function.

Synthesis – Covariation between drought, herbivory and economic traits means that, of the many potential trait combinations, few actually exist within these grasslands. Consequently, changes in land management and climate should have predictable effects on drought resistance, forage quality and productivity in the western Great Plains. 

Methods
Sampling methods-general: We sampled 5-10 replicate individual plants per species, depending upon the trait. For most traits, we sampled during flowering, thereby standardizing each measurement by plant developmental stage. For leaf osmotic potential, which can vary as water availability changes within a growing season, we constrained our sampling campaigns to 3-4 week periods of favorable soil moisture conditions when species diversity was at its seasonal maximum.

The majority of traits were measured in mixedgrass prairie in 2013 and shortgrass steppe in 2014. Exceptions were leaf senescence date, leaf pubescence, plant height (measured in 2015 at both sites), and leaf osmotic potential (measured in mixedgrass in 2015 and shortgrass in 2017).